I'm going to try to explain this as best as I can. I am still confused about it as he can't explain well.
He made this list in his head;
1- 9
2- 8
3- 7
4- 6
5- 5
6- 4
7- 3
8- 2
9- 1
So lets say he's doing 7+3.
He'll take the lowest number (3) and turn it into a teen (13). So it would look like 13+7.
But then he'll use his list and take the opposite of the other number and replace it. So 7 would be 3 and vice versa. 2 would be 8 and vice versa.
So then the problem would look like 13+3. But then he'll flip the sign into a subtraction symbol and do the problem and get the answer.
But this formula only seems to work with the numbers between 1-9. So someone tell me if this has been an established formula or if my six year old brother is a genius?
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Edited by STARSIDE DESTINY: 2/22/2014 6:02:25 PMBasically you just add 10 to the problem to start. Then when you use the list, you can see that each combination adds up to 10. By switching the numbers using the list and then subtracting that number, you're just subtracting 10 from the original problem. It's still pretty cool though, congrats to your brother. EDIT: Other people already pointed out how it works, I didn't read the comments before I posted.